The response of ionospheric convection in the polar cap to substorm activity

We report multi-instrument observations during an isolated substorm on 17 October 1989. The EISCAT radar operated in the SP-UK-POLI mode measuring ionospheric convection at latitudes 71°-78°. SAMNET and the EISCAT Magnetometer Cross provide information on the timing of substorm expansion phase onset and subsequent intensifications, as well as the location of the field aligned and ionospheric currents associated with the substorm current wedge. IMP-8 magnetic field data are also included. Evidence of a substorm growth phase is provided by the equatorward motion of a flow reversal boundary across the EISCAT radar field of view at 2130 MLT, following a southward turning of the interplanetary magnetic field (IMF). We infer that the polar cap expanded as a result of the addition of open magnetic flux to the tail lobes during this interval. The flow reversal boundary, which is a lower limit to the polar cap boundary, reached an invariant latitude equatorward of 71° by the time of the expansion phase onset. A westward electrojet, centred at 65.4°, occurred at the onset of the expansion phase. This electrojet subsequently moved poleward to a maximum of 68.1° at 2000 UT and also widened. During the expansion phase, there is evidence of bursts of plasma flow which are spatially localised at longitudes within the substorm current wedge and which occurred well poleward of the westward electrojet. We conclude that the substorm onset region in the ionosphere, defined by the westward electrojet, mapped to a part of the tail radially earthward of the boundary between open and closed magnetic flux, the “distant” neutral line. Thus the substorm was not initiated at the distant neutral line, although there is evidence that it remained active during the expansion phase. It is not obvious whether the electrojet mapped to a near-Earth neutral line, but at its most poleward, the expanded electrojet does not reach the estimated latitude of the polar cap boundary.     

Study of the longitudinal expansion velocity of the substorm current wedge

In this work we examine simultaneous observations from the two geosynchronous satellites GOES-5 and GOES-6 located at 282°E and 265°E respectively, and from middle and low latitude ground observatories located within 250°E and 294°E geographic longitude, during isolated substorms of moderate activity. The spatial distribution of our observation points allows us to make a detailed study of the azimuthal expansion of the substorm current wedge. The data analysis shows evidence that the substorm initiation and development mechanism include the cross-tail current diversion/disruption, the substorm current wedge formation and the azimuthal expansion of the inner plasma sheet. The triggering mechanism is initially confined in a longitudinally narrow sector, estimated to be less than 15° and located very close to local midnight to the east or to the west. The current disruption region expands both eastward and westward in the magnetotail, so that the location of major field-aligned currents flowing into the ionosphere shifts successively eastward, and the location of the currents flowing out of the ionosphere shifts successively westward. Evidence was found that the perturbation travels toward the west with velocities greater than those expanding the wedge eastward. The drastic decrease of the velocity with the azimuthal distance from the location of the disturbance initiation, i.e., the onset sector, indicates that the energy release is a very localized phenomenon. Finally, the transient D perturbation observed by the geosynchronous satellites suggests that the field-aligned currents forming the wedge have a longitudunally limited extent.     

Events of August 2, 2002: Features of substorm field-aligned currents

A model of alignment of the field-aligned current generator system in the disturbed magnetosphere is proposed. It has been found that Pedersen currents in the disturbed ionosphere of the auroral oval are meridional. They connect oppositely directed field-aligned currents (FACs) of two adjacent Iijima and Potemra (IP) regions. This supplements the dominant concept of currents in the literature, in which a substorm current wedge plays a major role. During the considered substorm, graphs have been obtained of changes in FAC intensities in each of the three Iijima and Potemra regions of the Northern Hemisphere. These new data suggest that during a disturbance, the Region 1 FAC and the sum of regions 2 and 0 FACs remain equal. The equality means that these three generators form a unified system. Inequalities of intensities between two FACs of different sign simultaneously observed in the dawn and dusk sectors have also been detected in each IP region. A conceptual model is proposed for the formation of this dawn-dusk asymmetry. In the model, in addition to FACs that close up in the ionosphere of the Northern Hemisphere, FACs connecting the two hemispheres are also presented. The conclusion is justified that the global magnetospheric dynamo operates as a voltage generator under relatively quiet (average) conditions, as during storms.     

Distribution of the field−aligned currents in the ionosphere: dawn–dusk asymmetry and its relation to the asymmetry between the two hemispheres

The dynamics of the system of field-aligned currents (FACs) and closing ionospheric Pedersen currents is considered with the use of original processing methods and the data from four substorms of different types. The total current system comprises of two parts. One is the well-known substorm current wedge (SCW) system, in which the zonal (westward ) current closes FACs in the R1 zone (region). The component 2 consists of two pairs of meridional currents flowing equatorward and poleward in the R1 region and creating regions R0 and R2 (according to the classification of Iijima and Potemra). It is shown that the total FAC of the disturbed magnetosphere–ionosphere system is dominated by the contribution of component 2, which contradicts the original version of the SCW model but is consistent with new data. The quantitative characteristics of the dawn–dusk asymmetry are determined for the FAC distribution in the ionosphere for each substorm. It is shown that the ratio of the average intensities of FACs in the regions R0 and R2 was I_R0/I_R2 ≥ 0.4, which contradicts the popular opinion that there are no FACs in the polar cap. In addition, a relatively rare event of the simultaneous start of the substorm explosive phase and the SSC caused by the dynamic impact of the solar wind when the polar cap expands (rather than compresses as usual) is considered.     

Evidence and effects of a wave-driven nonlinear current in the equatorial electrojet

Ionospheric two-stream waves and gradientdrift waves nonlinearly drive a large-scale (D.C.) current in the E-region ionosphere. This current flows parallel to, and with a comparable magnitude to, the fundamental Pedersen current. Evidence for the existence and magnitude of wave-driven currents derives from a theoretical understanding of E-region waves, supported by a series of nonlinear 2D simulations of two-stream waves and by data collected by rocket instruments in the equatorial electrojet. Wave-driven currents will modify the large-scale dynamics of the equatorial electrojet during highly active periods. A simple model shows how a wave-driven current appreciably reduces the horizontally flowing electron current of the electrojet. This reduction may account for the observation that type-I radar echoes almost always have a Doppler velocity close to the acoustic speed, and also for the rocket observation that electrojet regions containing gradientdrift waves do not appear also to contain horizontally propagating two-stream waves. Additionally, a simple model of a gradient-drift instability shows that wavedriven currents can cause nonsinusoidal electric fields similar to those measured in situ.     

SABRE observations of Pi2 pulsations: case studies

The characteristics of substorm-associated Pi2 pulsations observed by the SABRE coherent radar system during three separate case studies are presented. The SABRE field of view is well positioned to observe the differences between the auroral zone pulsation signature and that observed at mid-latitudes. During the first case study the SABRE field of view is initially in the eastward electrojet, equatorward and to the west of the substorm-enhanced electrojet current. As the interval progresses, the western, upward field-aligned current of the substorm current wedge moves westward across the longitudes of the radar field of view. The westward motion of the wedge is apparent in the spatial and temporal signatures of the associated Pi2 pulsation spectra and polarisation sense. During the second case study, the complex field-aligned and ionospheric currents associated with the pulsation generation region move equatorward into the SABRE field of view and then poleward out of it again after the third pulsation in the series. The spectral content of the four pulsations during the interval indicate different auroral zone and mid-latitude signatures. The final case study is from a period of low magnetic activity when SABRE observes a Pi2 pulsation signature from regions equatorward of the enhanced substorm currents. There is an apparent mode change between the signature observed by SABRE in the ionosphere and that on the ground by magnetometers at latitudes slightly equatorward of the radar field of view. The observations are discussed in terms of published theories of the generation mechanisms for this type of pulsation. Different signatures are observed by SABRE depending on the level of magnetic activity and the position of the SABRE field of view relative to the pulsation generation region. A twin source model for Pi2 pulsation generation provides the clearest explanation of the signatures observed.     

Method of characteristics in spherical geometry applied to a Harang-discontinuity situation

The method of characteristics for obtaining spatial distributions of ionospheric electrodynamic parameters from ground-based spatial observations of the ground magnetic disturbance and the ionospheric electric field is presented in spherical geometry. The method includes tools for separation of the external magnetic disturbance, its continuation to the ionosphere, and calculation of ionospheric equivalent currents. Based on these and the measured electric field distribution, the ionospheric Hall conductance is calculated as the primary output of the method. By estimating the Hall- to-Pedersen conductance ratio distribution, the remaining ionospheric electrodynamic parameters are inferred. The method does not assume = 0 to allow to study time-dependent situations. The application of this method to a Harang discontinuity (HD) situation on 27 October 1977, 17:39 UT, reveals the following: (1) The conductances at and north of the HD are clearly reduced as compared to the eastern electrojet region. (2) Plasma flow across the HD is observed, but almost all horizontal current is diverted into upward-flowing field-aligned currents (FACs) there. (3) The FACs connected to the Hall currents form a latitudinally aligned sheet with a magnitude peak between the electrically and magnetically defined HD, where break-up arcs are often observed. Their magnitude is larger than that of the more uniformly distributed FACs connected to the Pedersen currents. They also cause the southward shift of the magnetically defined HD with respect to the electrically defined one. (4) A tilt of the HD with respect to geomagnetic latitude as proposed by an earlier study on the same event, which used composite vector plot technique, and by statistical studies, is not observed in our single time-step analysis.Also at: Finnish Meteorological Institute, Geophysical Research, P.O. Box 503, FIN-00101 Helsinki, Finland.     

Electric circuits and their generators in the earth’s magnetosphere: The concept of electric circuits as applied to the first phase of the April 6, 2000, superstorm

{1} The first phase of the superstorm on April 6, 2000 was studied based on the analogy between systems of magnetospheric currents and wire electric currents. The conventional dataset supplemented with maps of ionospheric equivalent currents (ECs) and field-aligned currents (FACs) was also used. The application of this analogy made it possible to introduce spatial R.N inhomogeneities into FAC distributions in the two-dimensional ionosphere and three types of meridional current systems, MCS-0, MCS-1, and MCS-2, providing electric coupling of three Iijima and Potemra FAC Regions. This basis was used to describe the formation and observed dynamics of ionospheric auroral electrojets and three-dimensional current systems in a disturbed magnetosphere-ionosphere system. The results the modify known paradigms of the substorm current wedge (SCW). A new important fact was noted: simultaneously with the beginning of the disturbance expansion phase due to the stepwise growth in the dynamic pressure of the solar wind (SW), the stepwise growth in the area of polar cap and in the electromagnetic energy flux coming to the magnetosphere from the SW were observed.     

Active phase of a substorm as a chain of two types of reconnection: In the closed plasma sheet and in the open tail lobes

The maps of the field-aligned current (FAC) density distribution in the ionosphere obtained by the TIM-2 magnetogram inversion technique are used to investigate the August 27, 2001 substorm. The open magnetic flux Ψ and intensity J of the substorm current wedge (SCW) have been determined with a step of 1–5 min. The substorm onsets are divided into two types, PSR (plasma sheet reconnection) and TLR (tail lobe reconnection). The fast expansion tailward of the PSR region is described as the transition from PSR to PSR+TLR. Assuming that the SCW FACs flow down into the ionosphere from the edges of the disruption region of the cross-tail dawn-dusk current, several parameters of the disruption region have been estimated. The disrupted magnetic field has been found to be ∼5% of the undisrupted one for PSR and ∼95% for PSR+TLR. The disturbance power Q for PSR is an order of magnitude lower than that for PSR+TLR. The abrupt growth of Q during the transition from PSR to PSR+TLR is observed over the entire SCW area from its near-Earth part to the midtail and distant tail.     

1998年8月6日亚暴期间极光电集流指数AE的特征分析

极光电集流指数(AE)被广泛用来定量描述由电离层电流引起的极光带地磁活动.AE指数也是宏观描述亚暴事件强度和发展过程的一个主要指标,它的优点在于简单直观和快速.本文对1998年8月6日系列亚暴事件的极光活动、电流体系和AE指数的形态特征进行了分析.结果表明,伪亚暴期间虽然AE指数大于500 nT 且有快速增长和缓慢恢复的形态,但是极光没有明显的极向或者赤道向的扩展,极区电流体系主要呈现对流特征;伪亚暴和亚暴期间AE指数的形态变化没有本质区别,但电集流中心的位置有很大差异.我们进一步分析了台站分布对AE指数计算的影响,探讨了伪亚暴和亚暴期间磁场扰动最大值所处的磁地方时的差异.     

Multiple current sheets in a double auroral oval observed from the MAGION-2 and MAGION-3 satellites

A case is described of multiple current sheets crossed by the MAGION-2 satellite in the near-midnight quieting auroral oval. The data were obtained by the magnetometer experiment onboard. Results show during a quieting period after a preceding substorm, or during an early growth phase of the next substorm, two double-sheet current bands, POLE and EQUB, located at respectively the polar and equatorial borders of the auroral oval separated by about 500 km in latitude. This is consistent with the double-oval structure during recovery introduced by Elphinstone et al. (1995). Within the POLE, the magnetic field data show simultaneous existence of several narrow parallel bipolar current sheets within the upward current branch (at 69.5–70.3° invariant latitude) with an adjacent downward current branch at its polar side at (70.5–71.3°). The EQUB was similarly stratified and located at 61.2–63.5° invariant latitude. The narrow current sheets were separated on average by about 35 km and 15 km, respectively, within the POLE and EQUB. A similar case of double-oval current bands with small-scale structuring of their upward current branches during a quieting period is found in the data from the MAGION-3 satellite. These observations contribute to the double-oval structure of the late recovery phase, and add a small-scale structuring of the upward currents producing the auroral arcs in the double- oval pattern, at least for the cases presented here. Other observations of multiple auroral current sheets and theories of auroral arc multiplicity are briefly discussed. It is suggested that multiple X-lines in the distant tail, and/or leakage of energetic particles and FA currents from a series of plasmoids formed during preceding magnetic activity, could be one cause of highly stratified upward FA currents at the polar edge of the quieting double auroral oval.     

The high latitude convection response to an interval of substorm activity

On 17 March 1991, five clear substorm onsets/intensifications took place within a three hour interval. During this interval ground-based data from the EISCAT incoherent scatter radar, a digital CCD all sky camera, and an extensive array of magnetometers were available, in addition to data from the CRRES and DMSP spacecraft, whose footprints passed over Scandinavia very close to most of the ground-based instrumentation. This interval of substorm activity has been interpreted as being in support of a near-Earth current disruption model of substorm onset. In the present study the ionospheric convection response, observed some four hours to the west in MLT by the Halley HF radar in Antarctica, is related to the growth, expansion and recovery phases of two of the substorm onsets/expansions observed in the Northern Hemisphere. Bursts of ionospheric flow and motion of the convection reversal boundary (CRB) are observed at Halley in response to the substorm activity and changes in the IMF. The delay between the substorm expansion phase onset and the response in the CRB location is dependent on the local time separation from, and latitude of, the initial substorm onset region. These results are interpreted in terms of a synthesis of the very near-Earth current disruption model and the near-Earth neutral line model of substorm onset.     

A study on three-dimensional structures of the ionospheric dynamo currents induced by the neutral winds simulated by the Kyushu-GCM

Three-dimensional structures of the ionospheric dynamo currents are examined using the neutral winds in a general circulation model of the middle atmosphere at Kyushu University. A quasi-three-dimensional ionospheric dynamo model is constructed assuming an infinite parallel conductivity in the ionosphere. This model is able to simulate both the equatorial electrojet and the global Sq current system successfully. The simulated results reveal that the equatorial electrojet is confined in quite narrow latitudes around the equator accompanied with meridional current circulations and satisfies a non-divergent structure mainly within the E region. A vertically stratified double layered structure is seen in the east–west current density near the focus latitude of the global Sq current system. It is shown that the stratified structure mainly consists of the east–west Hall current associated with the eastward wind of zonal wavenumbers 1 and 2 in the lower altitudes and the westward wind of zonal wavenumber 2 in the upper altitudes. The day-to-day variation of the neutral winds can significantly vary the induced ionospheric dynamo current system, which is recognized as changes of the focus latitude and/or the maximum value of the equatorial electrojet.     

Westward moving dynamic substorm features observed with the IMAGE magnetometer network and other ground-based instruments

We present the ground signatures of dynamic substorm features with particular emphasis on the event interpretation capabilities provided by the IMAGE magnetometer network. This array covers the high latitudes from the sub-auroral to the cusp/cleft region. An isolated substorm on 11 Oct. 1993 during the late evening hours exhibited many of well-known features such as the Harang discontinuity, westward travelling surge and poleward leap, but also discrete auroral forms, known as auroral streamers, appeared propagating westward along the centre of the electrojet. Besides the magnetic field measurements, there were auroral observations and plasma flow and conductivity measurements obtained by EISCAT. The data of all three sets of instruments are consistent with the notion of upward field-aligned currents associated with the moving auroral patches. A detailed analysis of the electro-dynamic parameters in the ionosphere, however, reveals that they do not agree with the expectations resulting from commonly used simplifying approximations. For example, the westward moving auroral streamers which are associated with field-aligned current filaments, are not collocated with the centres of equivalent current vortices. Furthermore, there is a clear discrepancy between the measured plasma flow direction and the obtained equivalent current direction. All this suggests that steep conductivity gradients are associated with the transient auroral forms. Also self-induction effects in the ionosphere may play a role for the orientation of the plasma flows. This study stresses the importance of multi-instrument observation for a reliable interpretation of dynamic auroral processes.     

Quantitative analysis of the relationship between polar ionospheric currents and auroral electrojet indices

Magnetic storms and substorms are principalprocesses of energy transition from the solar wind intothe magnetosphere-ionosphere system and dissipationin the system. They are also important events whichthe space physics study and space weather predictionhave been focused on. Magnetic storms are describedby means of the magnetic index Dst, which is calcu-lated using the magnetic disturbances of horizontalcomponent recorded at 5 low-latitudinal stations, rep-resenting approximately the symmetric r…     

Poleward expansion of the westward electrojet depending on the solar wind and IMF parameters

The effect of the interplanetary parameters on the latitudinal position of the substorm westward electrojet is studied in the work. The data from the IMAGE chain of magnetic stations and POLAR and WIND satellites for the period close to the solar activity minimum (1995–1996) and for the period of the solar activity maximum (2000) have been used for this purpose. It has been indicated that the electrojet poleward edge reaches, on average, higher latitudes at a higher solar wind velocity and at a larger (B _s ) IMF southward component. It has been indicated that the average latitude of the westward electrojet center increases with increasing solar wind velocity and decreases with increasing IMF southward component, as a result of which the electrojet center is, specifically, not observed at high geomagnetic latitudes at large values of the IMF southward component.     

Relation of auroral substorm onset to local AL index and dispersionless particle injections

In this study, we investigate the relation of auroral substorm onset to the sharp decrease in the local AL index (IL) during substorms. With a database of over 4200 onsets determined from auroral images, we have statistically examined the timing between the auroral substorm onset and the sharp decrease in the IL index, as determined with data from the IMAGE magnetometer network. From the database of onsets, 54 substorms were determined to be within 6° of the central meridian of the IMAGE ground array. Our superposed epoch median curve shows that the IL index begins to sharply decrease 3 min before the auroral onset, which is twice the 2 min resolution of the auroral imager. However, the mean difference determined by measuring the time between the start of the IL decrease and the auroral substorm onset is about 1.1±0.6 min. An analysis of the superposed epoch median curves of the SOPA particle data for the LANL spacecraft closest to the auroral onset meridian indicates that both the electron and proton injections begin about 3 min before the auroral onset. However, the mean time of the difference between the minimum of the particle dispersionless injection and the auroral onset is simultaneous within the uncertainty of the auroral onset and the error of the mean for the injection. The location of the electron injection relative to the IMAGE ground array seems to be 01–04 MLT, while the proton injection appears to be in the 22–01 MLT sector. These statistical results support the idea that the field aligned and ionospheric currents of the substorm current wedge begin to flow before the auroral onset.     

Models of field-aligned currents needful to simulate the substorm variations of the electric field and other parameters observed by EISCAT

We have used the global numerical model of the coupled ionosphere-thermosphere-protonosphere system to simulate the electric-field, ion- and electron-temperature and -concentration variations observed by EISCAT during the substorm event of 25 March 1987. In our previous studies we adopted the model input data for field-aligned currents and precipitating electron fluxes to obtain an agreement between observed and modelled ionospheric variations. Now, we have calculated the field-aligned currents needful to simulate the substrom variations of the electric field and other parameters observed by EISCAT. The calculations of the field-aligned currents have been performed by means of numerical integration of the time-dependent continuity equation for the cold mag-netospheric electrons. This equation was added to the system of the modelling equations including the equation for the electric-field potential to be solved jointly. In this case the inputs of the model are the spatial and time variations of the electric-field potential at the polar-cap boundaries and those of the cold magnetospheric electron concentration which have been adopted to obtain the agreement between the observed and modelled ionospheric variations for the substorm event of 25 March 1987. By this means it has been found that during the active phase of the substorm the current wedge is formed. It is connected with the region of the decreased cold magnetospheric electron content travelling westwards with a velocity of about 1 km s^–1 at ionospheric levels.     

Substorm fields in and near the auroral zone

During the last few years, the study of both temporal and spatial variations of substorm fields has rapidly expanded, mainly because of the relationships which exist between polar magnetic substorms and magnetospheric phenomena. Also during these years, proposed current systems believed to be responsible for substorm variations have evolved into complex three-dimensional systems with field-aligned and magnetospheric currents coupled to the eastward and westward electrojets. Recent model studies show that substorm variations in and near the auroral zone can easily be modelled using both two and three-dimensional current systems. In these studies, induction effects were simulated by assuming the Earth to be infinitely conducting at some depth below the surface.The use of magnetometers distributed along magnetic meridians has resulted in a better understanding of the complex current patterns making up the electrojets. For example, during the expansive phase of substorms, the westward and poleward progression of the overall westward electrojet was discovered to take place through the sequential development of a series of westward electrojets.     

Combined CUTLASS, EISCAT and ESR observations of ionospheric plasma flows at the onset of an isolated substorm

On August 21st 1998, a sharp southward turning of the IMF, following on from a 20 h period of northward directed magnetic field, resulted in an isolated substorm over northern Scandinavia and Svalbard. A combination of high time resolution and large spatial scale measurements from an array of coherent scatter and incoherent scatter ionospheric radars, ground magnetometers and the Polar UVI imager has allowed the electrodynamics of the impulsive substorm electrojet region during its first few minutes of evolution at the expansion phase onset to be studied in great detail. At the expansion phase onset the substorm onset region is characterised by a strong enhancement of the electron temperature and UV aurora. This poleward expanding auroral structure moves initially at 0.9 km s^-1 poleward, finally reaching a latitude of 72.5°. The optical signature expands rapidly westwards at ~6 km s^-1, whilst the eastward edge also expands eastward at ~0.6 km s^-1. Typical flows of 600 m s^-1 and conductances of 2 S were measured before the auroral activation, which rapidly changed to ~100 m s^-1 and 10–20 S respectively at activation. The initial flow response to the substorm expansion phase onset is a flow suppression, observed up to some 300 km poleward of the initial region of auroral luminosity, imposed over a time scale of less than 10 s. The high conductivity region of the electrojet acts as an obstacle to the flow, resulting in a region of low-electric field, but also low conductivity poleward of the high-conductivity region. Rapid flows are observed at the edge of the high-conductivity region, and subsequently the high flow region develops, flowing around the expanding auroral feature in a direction determined by the flow pattern prevailing before the substorm intensification. The enhanced electron temperatures associated with the substorm-disturbed region extended some 2° further poleward than the UV auroral signature associated with it.